Download pdf - Question 1 Given a battery and a light bulb, show how you would connect these two devices

Question 1

Given a battery and a light bulb, show how you would connect these two devices together with wire soas to energize the light bulb:

+-

file 00001

Answer 1

This is the simplest option, but not the only one.

+-

Notes 1

This question gives students a good opportunity to discuss the basic concept of a circuit. It is veryeasy to build, safe, and should be assembled by each student individually in class. Also, emphasize howsimple circuits like this may be assembled at home as part of the ”research” portion of the worksheet. Toresearch answers for worksheet questions does not necessarily mean the information has to come from abook! Encourage experimentation when the conditions are known to be safe.

Have students brainstorm all the important concepts learned in making this simple circuit. What generalprinciples may be derived from this particular exercise?

1

Question 2

Write the three forms of Ohm’s Law (E = · · · , I = · · · and R = · · ·), showing how voltage, current,and resistance relate to one another:

file 03336

Answer 2

E = IR I = ER

R = EI

Notes 2

This question is intended for exams only and not worksheets!.

2

Question 3

Calculate the voltage dropped across a 1.8 kΩ resistor and the power dissipated by that resistor if it isconducting 20 mA of current.

E = P =file 03344

Answer 3

E = 36 volts P = 0.72 watts

Notes 3


3

Question 4

Identify whether or not there should be voltage between the specified pairs of test points in this simplecircuit:

A B C D E

F G H I J

• Between C and H – voltage or no voltage?• Between A and B – voltage or no voltage?• Between I and J – voltage or no voltage?• Between A and F – voltage or no voltage?

file 03353

Answer 4

• Between C and H – voltage• Between A and B – no voltage• Between I and J – no voltage• Between A and F – voltage

Notes 4


4

Question 5

Choose the proper band colors for a 4-band resistor (assume a tolerance of 5%) that will let 3.7 mA ofcurrent through it with 20.72 volts dropped across it:

file 03346

Answer 5

Green Blue Red Gold

Notes 5


5

Question 6

Calculate the amount of current through a 240 volt DC electric motor as it draws 2.3 kW of electricpower from the circuit source:

Mtr240 V

I = ???

2.3 kWP =

file 03349

Answer 6

I = 9.5833 amps

Notes 6


6

Question 7

Calculate the resistor voltage (VR) and resistor current (IR) in this circuit:

5 volts 12 volts 10 volts

R = 2.2 kΩ

VR = IR =file 03370

Answer 7

VR = 7 volts IR = 3.182 mA

Notes 7


7

Question 8

Suppose the motor in this circuit refused to run, despite your voltmeter showing that there is 36 voltsbetween points E and J at the battery:

A B C D E

F G H I J

Mtr 36 V

Connecting your voltmeter between points B and G in the circuit, you measure 36 volts as well. Whatdoes this indicate about the location of the fault? Assume there is only one thing wrong with this circuit(no multiple faults!).

file 03391

Answer 8

The fault must be somewhere to the left of points B and G.

Notes 8


8

Question 9

Determine the voltage that a digital voltmeter would register if connected between the specified pointsin this circuit:

25 V

5 V

14 V

6 V 7 V

3 V

4 V

A B C

D E F

Between points A (red lead) and C (black lead):

Between points E (red lead) and C (black lead):file 03374

Answer 9

Between points A (red lead) and C (black lead): +13 volts

Between points E (red lead) and C (black lead): -7 volts

Notes 9


9

Question 10

Calculate the current in this circuit (I), and also the approximate length of time that this 36 amp-hourbattery will be able to maintain power to the load before it dies (t):

12 V3.1 Ω

I = t =file 03404

Answer 10

I = 3.871 A t = 9.3 hours

Notes 10


10

Question 11

Identify the magnitudes and directions of the specified resistor currents in this circuit:

R1

R4

R6

R3 R2

R5

R7

R8200 mA

40 mA

110 mA

Note: all arrows point in the direction of conventional flow notation.

• IR1 = ; Direction = to the left or to the right?



• IR3 = ; Direction = up or down?

file 03419

Answer 11

• IR1 = 110 mA ; Direction = to the right

• IR5 = 240 mA ; Direction = to the left

• IR6 = 40 mA ; Direction = to the right

• IR3 = 130 mA ; Direction = down

Notes 11


11

Question 12

Calculate the value of R3 necessary to create a total resistance of 441 Ω:

R3 =

RT = 441 Ω 1 kΩR1 = R2 = R3 =

1.5 kΩ ???

Also, write an equation solving for the value of R3 in terms of RT , R1, and R2.file 03411

Answer 12

R3 = 1.664 kΩ

R3 =1

1RT

−(

1R1

+ 1R2

)

Notes 12


12

Question 13

The following electric heater seems to have a problem: it heats up slower than usual with all threeswitches turned ”on.”

Fuse

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Oven

200 W

400 W

800 W

240 volts

With three differently-sized heating elements (200 watt, 400 watt, and 800 watt), the oven operator canset the power in seven discrete steps by turning on specific combinations of switches: 200 watts, 400 watts,600 watts, 800 watts, 1000 watts, 1200 watts, and 1400 watts.

You are summoned to diagnose this oven’s problem without turning it off. You are allowed to turn offany single switch for a few seconds at most, but otherwise you need to leave all three heaters on because theoven needs to heat up as fast as it can! The idea is to figure out where the problem might be, then gathertogether any parts necessary for repairs while the oven is still being used, and fix the oven as fast as possiblewhen you finally get the chance to turn it off completely.

Using a magnetic ”clamp-on” ammeter to measure current without breaking the circuit, you read 4.1amps through the wire between the power plug and terminal 13 with all three switches in the ”on” position.Then, you momentarily turn the ”800 watt” switch off and on, watching the current fall from 4.1 amps to0.8 amps and then return to 4.1 amps.

Based on this data, identify two things:

• Two components or wires in the oven circuit that you know must be in good working condition.

• Two components or wires in the oven circuit that could possibly be bad (and thus cause the slow heatingproblem).

file 03414

13

Answer 13

The data suggests something failed open in the 400 watt heater circuit. All wires and componentsconducting current to the 200 watt and 800 watt heaters must be good.

Notes 13


14

Question 14

Calculate the value of resistor R3 needed to balance this bridge circuit, given the following resistancevalues:

+V

-

R1

R2

R3

R4

• R1 = 1.9 kΩ

• R2 = 2.5 kΩ

• R3 =

• R4 = 1 kΩ

Also, write an equation showing how these four resistor values relate to one another in a condition ofbalance.

file 03429

Answer 14

R3 = 760 Ω R1

R2

= R3

R4

Notes 14


15

Question 15

Qualitatively determine what will happen to the currents through resistors R1 and R2 if the switchconducting power to the load is suddenly opened (turned off):

LoadR1

R2

Switch

• IR1 will increase, decrease, or stay the same?• IR2 will increase, decrease, or stay the same?

file 03431

Answer 15

• IR1 will increase• IR2 will decrease

Notes 15


16

Question 16

Calculate the total resistance of this network with the potentiometer in minimum and maximumresistance positions (as shown in these two diagrams):

2 kΩ 1 kΩMinimum R

(0 Ω)

1.5 kΩ

2 kΩ 1 kΩ

1.5 kΩ

Maximum R

(10 kΩ)

Rtotal (min.) = ??? Rtotal (max.) = ???

file 03439

Answer 16

Rtotal (min.) = 2.6 kΩ Rtotal (max.) = 3.32 kΩ

Notes 16


17

Question 17

Identify the North and South poles on this electromagnet. Also draw the magnetic lines of flux, showingtheir paths:

file 03479

Answer 17

I I

NS

Notes 17


18

Question 18

A piece of laboratory equipment uses a voltage divider to reduce voltage to two electromagnet coils froma higher-voltage source. Coil #1 is supposed to receive 2.91 volts and coil #2 is supposed to receive 13.11volts:

150 Ω 175 Ω

R1

R2

R3 Coil 1 Coil 2200 Ω

300 Ω

100 Ω

24 V

TP1

TP2

TP3

TP4TP5

One day, something goes wrong with this circuit. The magnetic field from coil #1 suddenly disappears,yet there is still a magnetic field coming from coil #2. The technician who looked at this problem beforeyou took two voltage measurements and then gave up: 13.55 volts at test point TP3 and 5.42 volts at testpoint TP4. You left your multimeter back at the shop, which means you cannot take any more voltagemeasurements. However, since you are more determined than the former technician, you proceed to identifythe following from the two measurements already taken:

• Two components or wires in the circuit that you know cannot be failed either open or shorted, besidesthe 24 volt source which is obviously operational.

• One component or wire in the circuit you think could possibly be bad, and the type of failure it wouldbe (either open or shorted).

file 03487

Answer 18

Components known to be in good working condition:

• Wire from battery to R1

• Wire from TP2 to TP3• R1

• R2

• R3

• Coil #2

Components which could possibly be faulted:

• Wire from TP4 to TP5 broken (open)• Coil #1 failed open

19

Notes 18


20

Question 19

Identify two different component values that could be changed in this circuit to make the time constantlonger (creating a slower circuit response):

R

V L

Be sure to identify both the components and the directions their respective values must be changed(larger or smaller).

file 03519

Answer 19

Make R smaller. Make L larger.

I recommend assigning two points to each properly identified component, and two points for each properdirection of change.

Notes 19


21

Question 20

Calculate the total inductance in this network:

10 mH

25 mH

15 mH

Ltotal = ???

file 03528

Answer 20

Ltotal = 4.839 mH

Notes 20


22

Question 21

Calculate the voltage across a 47 µF capacitor after charging from a 12 volt source through a totalcircuit resistance of 22 kΩ for 2 seconds.

VC =file 03540

Answer 21

VC = 10.27 V

Notes 21


23

Question 22

In this time-delay relay circuit, the motor will immediately start when the pushbutton is pressed, andcontinue to run for about 5 seconds after the pushbutton is released. The green light-emitting diode (LED)is supposed to be on whenever the motor is stopped, and off whenever the motor is running:

C1

Pushbutton switchRelay R1

R2

Mtr LED24 V

TP1

TP2

TP3

TP4

TP5

TP6

TP7TP8TP9

However, a problem has developed with this circuit. Neither the motor nor the green LED ever energizes,no matter what is done with the pushbutton switch. Strangely enough, the relay does make a ”click” soundwhen the pushbutton is pressed, and then another ”click” sound about 5 seconds after the switch is released.Based on this information, determine the following:

• Two components or wires in the circuit that you know cannot be failed either open or shorted, besidesthe 24 volt source.

• Two components or wires in the circuit you think could possibly be bad (either one independentlycapable of causing the problem), and the type of failure each would be (either open or shorted).

file 03534

Answer 22


• Pushbutton switch• Capacitor• Relay coil


• Wire from TP1 to TP4 broken (open)• Wire from TP4 to relay broken (open)• Wire from TP7 to TP8 broken (open)• Relay reed burnt so it cannot contact either ”throw.”

Notes 22


24

Question 23

Determine the following parameters of this AC voltage waveform, assuming a vertical sensitivity of 0.5volts per division and a timebase of 1 millisecond per division:

V (peak) =

V (RMS) =

Period =

Frequency (f) =

file 03562

Answer 23

Vpeak = 1.5 volts VRMS = 1.061 volts

f = 200 Hz Period = 5 milliseconds

Notes 23


25

Question 24

Calculate total impedance (Ztotal), total current (Itotal), resistor current (IR), and inductor current (IL)in the following circuit:

4k7 22m3 V25 kHz

Ztotal =

Itotal =

IL =

IR =

Note: your answers may be simply scalar quantities (no polar or rectangular form necessary)file 03579

Answer 24

Ztotal = 2.784 kΩ

Itotal = 1.078 mA

IL = 868.1 µA

IR = 638.3 µA

Notes 24


26

Question 25

Determine (qualitatively) what will happen to the following voltages and current in this circuit asfrequency is increased (assume Vsupply remains constant):

R1

L1

R2

Vsupply

Itotal = increase, decrease, or stay the same?

VR1 = increase, decrease, or stay the same?

VL1 = increase, decrease, or stay the same?

VR2 = increase, decrease, or stay the same?file 03581

Answer 25

Itotal = decrease

VR1 = decrease

VL1 = increase

VR2 = decrease

Notes 25


27

Question 26

Determine the following parameters of these AC voltage waveforms, assuming a vertical sensitivity of 1volt per division and a timebase of 2 milliseconds per division:

Frequency (f) =

V1V2

V1 (peak) =

V2 (peak) =

Phase shift (θ) =

file 03598

Answer 26

V1 (peak) = 3 volts V2 (peak) = 3.4 volts

f = 100 Hz Θ = 72o

Notes 26


28

Question 27

Calculate the amount of reactance (XC) in ohms that the capacitor must have to give this circuit atotal impedance (Ztotal) of 1.7 kΩ, and also its capacitance value (C) in Farads if the source frequency is 2kHz:

720 Ω

XC = C =file 03609

Answer 27

XC = 1540 Ω C = 51.67 nF

Notes 27


29

Question 28

Determine (qualitatively) what will happen to the following parameters in this circuit as frequency isdecreased (assume Vsupply remains constant):

R1Vsupply

C1

C2

Itotal = increase, decrease, or stay the same?

VR1 = increase, decrease, or stay the same?

IC1 = increase, decrease, or stay the same?

Phase shift (Θ) between Vtotal and Itotal = increase (approach -90o), decrease (approach 0o), or stay thesame?

file 03606

Answer 28

Itotal = decrease

VR1 = stay the same

IC1 = decrease

Phase shift (Θ) between Vtotal and Itotal = decrease (approach 0o)

Notes 28


30

Question 29

A digital computer network uses a two-conductor cable to send both power (DC) and data (AC pulses)from one computer to another, inductors and capacitors being used to filter the voltages to and from theirproper destinations:

Digital datapulses (AC)

DC power

Digital data

DC powerCable

load

load

Sending computer Receiving computerC1

C2

C3

C4

L1

L2

L3

L4

V1

V2

The system works fine for a while, but then one day the receiving computer stops receiving data. It stillhas DC power getting to it, but just no digital data. Identify the following:

• One failed component in the circuit that could possibly account for the problem, and the type of fault(open or short) you suspect that component would have.

• Show where you would connect a voltmeter in the circuit to verify a fault in that one suspect component,and the voltage reading (AC, DC, or both) you would expect to get if indeed that one component hadfailed.

file 03603

Answer 29

Possible component faults, and their respective validating tests

• Source V1 failed (no signal); measure 0 AC voltage directly across its terminals.• Problem with receiving computer circuitry (digital data load ”resistor” failed open); measure full AC

voltage directly across its terminals.• Capacitor C1 failed open; measure full AC voltage directly across its terminals.• Capacitor C2 failed open; measure full AC voltage directly across its terminals.• Capacitor C3 failed open; measure full AC voltage directly across its terminals.• Capacitor C4 failed open; measure full AC voltage directly across its terminals.

Notes 29


31

Question 30

Explain what it means for an inductor-capacitor (LC) circuit to be in a state of resonance.

file 03616

Answer 30

Resonance is that state where the inductive and capacitive reactances are precisely equal to one another:

XL = XC

Notes 30


32

Question 31

Calculate the total impedance of this series RLC circuit and express it in polar form (magnitude andphase angle):

1 kHz0.15 µF

2.2 kΩ

100 mH

Ztotal =file 03626

Answer 31

Ztotal = 2.242 kΩ 6 -11.13o

I recommend assigning 4 points to the correct magnitude and 4 points to the correct angle.

Notes 31


33

Question 32

Identify whether each of these filter circuits is a low-pass, high-pass, band-pass, or band-stop:

Vin Vout

Vin Vout VoutVin

Vin Vout

file 03631

Answer 32

Vin Vout

Vin Vout VoutVin

Vin Vout

Band-passHigh-pass

Low-passBand-stop

Notes 32


34

Question 33

Plot the typical frequency responses of four different filter circuits, showing signal output (amplitude)on the vertical axis and frequency on the horizontal axis:

f

Vout

f

Vout

f

Vout

f

Vout

Low-pass

High-pass

Band-pass

Band-stop

Also, identify and label the bandwidth of the filter circuit on each plot.file 03949

35

Answer 33

f

Vout

f

Vout

f

Vout

f

Vout

Low-pass

High-pass

Band-pass

Band-stop

Bandwidth

Bandwidth

Bandwidth

Bandwidth

70.7%

70.7%

70.7%70.7%

70.7%70.7%

fc

fc

fc1 fc2

fc1 fc2

Notes 33


36

Question 34

A special type of component called a varactor diode is used to electronically tune resonant circuits, givenits ability to change capacitance with the application of a DC voltage. In the following schematic, the diode,capacitor, and inductor form a resonant circuit; the potentiometer creates a variable DC voltage to impressacross the varactor diode (to change its capacitance); and the radio-frequency choke (”RFC”) inductor actsas a low-pass filter to block AC from getting to the potentiometer:

RFC

L

Varactordiode

C

+V

Tuningcontrol

Resonant circuitTo rest of radio-frequency circuit

The equation for calculating the capacitance of a varactor diode is as follows:

Cdiode =Co

√2V + 1

Where,Cdiode = Diode capacitanceCo = Natural capacitance of diode with zero applied DC voltageV = Applied DC voltage

Based on this information, determine which direction the potentiometer must be moved to increase theresonant frequency of the tuning circuit shown, and also explain why this is so.

file 03632

Answer 34

Moving the pot wiper up increases the DC voltage across the varactor, which decreases its capacitance,which increases the resonant frequency of the LC circuit.

Notes 34


37

Question 35

Explain why transformers are used extensively in long-distance power distribution systems. Whatadvantage do they lend to a power system?

file 02213

Answer 35

Transformers are used to step voltage up for efficient transportation over long distances, and used tostep the high voltage down again for point-of-use circuits.

Notes 35

Ask your students to explain the answer in detail, rather than just repeating what the given answerstates. Why is high-voltage power distribution more efficient than low-voltage distribution? Why wouldhigh voltage have to be stepped down for point-of-use applications?

38

Question 36

Calculate the voltage output by the secondary winding of a transformer if the primary voltage is 35volts, the secondary winding has 4500 turns, and the primary winding has 355 turns.

Vsecondary =

file 02206

Answer 36

Vsecondary = 443.7 volts

Notes 36

Transformer winding calculations are simply an exercise in mathematical ratios. If your students arenot strong in their ratio skills, this question provides an application to sharpen them!

39

Question 37

Calculate the load current and load voltage in this transformer circuit:

Load45 VAC 2710 turns 930 turns

530 Ω

Iload = Vload =

file 03656

Answer 37

Iload = 29.14 mA Vload = 15.44 V

Notes 37


40

Question 38

The current feeding a large electric motor is measured by an ammeter connected to a current transformerwith a ratio of 200:5. If the ammeter current is 2.7 amps, how much is the motor current?

A

200:5 ratio

ACmotor

. . .

. . .

To AC powersource

CT

2.7 A

file 03659

Answer 38

Imotor = 108 amps

Notes 38


41

Question 39

Calculate the number of turns needed in the secondary winding of a transformer to transform a primaryvoltage of 480 volts down to a secondary voltage of 260 volts, if the primary winding has 1176 turns of wire.

Nsecondary =file 03661

Answer 39

Nsecondary = 637 turns

Notes 39


42

Question 40

A technician brings a control power transformer into your repair shop for diagnosis. It is a dual-voltageprimary unit, and was used on the job site to step 480 volts down to 120 volts:

H1 H2H3 H4

X1 X2

X1 X2

H1 H2H3 H4

240 × 480 primary

120 secondary

Schematic diagram

According to the person who did the field troubleshooting, the transformer is simply ”bad.” As is typical,you were given no other information to help diagnose the precise fault. Describe how you would test thistransformer in your shop for the following faults, noting the transformer terminals you would connect yourmultimeter to and the type of meter measurement you would expect to see for each specified fault.

First (left) primary winding failed open

• Meter connected between:

• Measurement expected for this type of fault:

Second (right) primary winding failed open



First (left) primary winding shorted to iron core



Secondary winding shorted to iron core


• Measurement expected for this type of fault:file 03667

43

Answer 40

First (left) primary winding failed open

• Meter connected between: H1 and H2

• Measurement expected for this type of fault: Infinite resistance

Second (right) primary winding failed open

• Meter connected between: H3 and H4

• Measurement expected for this type of fault: Infinite resistance

First (left) primary winding shorted to iron core

• Meter connected between: Either H1 or H2, and metal frame/core of transformer

• Measurement expected for this type of fault: Low resistance

Secondary winding shorted to iron core

• Meter connected between: Either X1 or X2, and metal frame/core of transformer

• Measurement expected for this type of fault: Low resistance

Notes 40


44

Question 41

An AC induction motor draws 18 amps of current at a voltage of 480 volts, with current lagging voltageby 30o. Calculate the true power consumed by the motor (P ), and also its power factor (P.F.):

P = P.F. =file 03680

Answer 41

P = 7.482 kW P.F. = 0.866, or 86.6%

Notes 41


45

Question 42

Draw a schematic diagram showing how the windings (coils) of a Wye three-phase AC generator areconnected together:

Then, calculate the line voltage if the phase voltage in this generator is 7.1 kV. Assume this generatoroperates in a balanced condition (all phase voltages and currents equal).

Vline =file 03683

Answer 42

Vline = 12.3 kV

Notes 42


46

Question 43

Calculate the following values in this balanced Delta-Wye system:

Source Load

100 Ω440 V

Vline =

Iline =

Vphase(load) =

Ptotal =

file 03689

Answer 43

Vline = 440 volts

Iline = 2.54 amps

Vphase(load) = 254 volts

Ptotal = 1.936 kW

Notes 43


47

Question 44

In this 480 volt AC induction motor control circuit, a three-pole relay (typically called a contactor) isused to switch power on and off to the motor. The contactor itself is controlled by a smaller switch, whichgets 120 volts AC from a step-down transformer to energize the contactor’s magnetic coil. Although thismotor control circuit used to work just fine, today the motor refuses to start.

Motor

motor

Fuses

Contactor

Contactor

H1

H2

H3

H4

X1

X2

Transformer

X1 X2

H1 H2H3 H4

SwitchSwitch

Transformer

To 3-φ , 480 volt power source

Schematic diagramL1 L2 L3L1 L2 L3

T1 T2 T3

T1 T2 T3

A1A2

A1

A2

Using your AC voltmeter, you measure 476 volts AC between L1 and L2, 477 volts AC between L2 andL3, and 475 volts AC between L1 and L3. You also measure 477 volts between transformer terminals H1and H4. With the switch in the ”on” position, you measure 0.5 volts AC between terminals X1 and X2 onthe transformer. From this information, identify the following:

• Two components or wires in the circuit that you know cannot be failed either open or shorted, besidesthe 480 volt AC source which is obviously operational.

• Two different component or wire failures in the circuit, either one of which could account for the problemand the measurement values, and the types of failures they would be (either open or shorted).

file 03688

48

Answer 44


• The center (L2) and right (L3) fuses• All wires from fuses to transformer primary


• Transformer primary winding(s) failed open• Transformer secondary winding failed open or failed shorted• Jumper wire from H2 to H3 failed open• Contactor coil failed shorted

Notes 44


49

Question 45

The following motor control circuit has a problem. When the ”Start” button is pressed, the motorrefuses to start:

M1

M1

Start StopM1

motor

To 3-phasepower source

L1 L2

OL

OL

Fuse 1

Fuse 2TP1

TP2

TP3

TP4

TP5

TP6

TP7

TP8

TP9 TP10 TP11 TP12

480/120 V

Using your digital voltmeter, you measure 120 volts AC between TP10 and Ground when the ”Start”switch is pressed, but 0 volts between the same points when the switch is released. From this information,identify two possible faults that could account for the problem and all measured values in this circuit, andalso identify two circuit elements that could not possibly be to blame (i.e. two things that you know mustbe functioning properly, no matter what else may be faulted). The circuit elements you identify as eitherpossibly faulted or properly functioning can be wires, traces, and connections as well as components. Be asspecific as you can in your answers, identifying both the circuit element and the type of fault.

• Circuit elements that are possibly faulted

1.

2.

• Circuit elements that must be functioning properly

1.

2.

file 03212

Answer 45

Note: the following answers are not exhaustive. There may be more circuit elements possibly at faultand more circuit elements known to be functioning properly!


1. Overload contact tripped (open)

2. ”Stop” switch contacts open

3. Contactor coil M1 failed open

50

• Circuit elements that must be functioning properly1. Control power transformer (480/120 volt unit supplying L1 and L2)2. Both fuses3. ”Start” switch contacts

Notes 45


51

Question 46

Trace the current through this rectifier circuit at a moment in time when the AC source’s polarity ispositive on right and negative on left as shown. Be sure to designate the convention you use for currentdirection (conventional or electron flow):

Rload

Also, mark the polarity of the voltage drop across Rload.file 02313

Answer 46

Current arrows shown in thedirection of conventional flow!

Notes 46

Have your students explain their reasoning when tracing currents and determining voltage drop polaritiesin front of the class for all too see. The bridge rectifier circuit is one that many students find confusing toanalyze, and so it is worth spending time on in class to fully understand.

52

Question 47

Qualitatively determine what will happen to the series resistor current and the zener diode current inthis voltage regulator circuit if the load current suddenly decreases. Assume that the zener diode’s behavioris ideal; i.e. its voltage drop holds absolutely constant throughout its operating range.

Rseries

Load

IRseries= (increase, decrease, or unchanged?)

Izener = (increase, decrease, or unchanged?)

file 02315

Answer 47

If the load current decreases, Izener will increase and IRserieswill remain unchanged.

Challenge question: what do you think will happen with a real zener diode, where its voltage drop doeschange slightly with changes in current?

Notes 47

A conceptual understanding of zener diode regulator circuits is important, perhaps even more importantthan a quantitative understanding. Your students will need to understand what happens to the differentvariables in such a circuit when another parameter changes, in order to understand how these circuits willdynamically react to changing load or source conditions.

53

Question 48

Calculate the approximate DC output voltage of this power supply when it is not loaded:

Vout = ???

115 VACRMS

1000turns

140turns

VF ≈ 0.7 volts(each)

file 02301

Answer 48

Vout ≈ 21.4 volts

Notes 48

Ask your students to explain how they solved for this output voltage, step by step.

54

Question 49

A technician is troubleshooting a power supply circuit with no DC output voltage. The output voltageis supposed to be 15 volts DC, but instead it is actually outputting nothing at all (zero volts):

Fuse

- -

TP1

TP3TP4

TP5

TP6TP7TP8

DC voltage output

TP9

TP10

TP2

T1

D1 D2

D3 D4

C1

The technician measures 120 volts AC between test points TP1 and TP3. Based on this voltagemeasurement and the knowledge that there is zero DC output voltage, identify two possible faults thatcould account for the problem and all measured values in this circuit, and also identify two circuit elementsthat could not possibly be to blame (i.e. two things that you know must be functioning properly, nomatter what else may be faulted). The circuit elements you identify as either possibly faulted or properlyfunctioning can be wires, traces, and connections as well as components. Be as specific as you can in youranswers, identifying both the circuit element and the type of fault.

• Circuit elements that are possibly faulted1.2.

• Circuit elements that must be functioning properly1.2.

file 02306

Answer 49

I’ll let you and your classmates figure out some possibilities here!

Notes 49

Troubleshooting scenarios are always good for stimulating class discussion. Be sure to spend plentyof time in class with your students developing efficient and logical diagnostic procedures, as this will assistthem greatly in their careers.

55

Question 50

Examine the following transistor symbol:

Identify the following:

• Type of transistor (BJT, JFET, or MOSFET)• Semiconductor doping (NPN, PNP ; N-channel, P-channel)• Identification of all 3 terminals (Base, Collector, Emitter ; Gate, Drain, Source)• Direction of each terminal’s current for proper transistor operation (be sure to note whether you are

using conventional flow or electron flow notation. If current direction is unimportant, or if there is nocurrent at all, be sure to say so!)

file 02328

Answer 50

BJT -- NPN

Emitter Collector

Base

Current arrows drawn in the directionof conventional flow notation

Notes 50

This question probes some basic yet important knowledge about transistor identify and operation. Ifstudents have difficulty identifying all the parameters asked for in this question, you need to spend moretime on transistor fundamentals before proceeding with any other aspects of transistor circuitry!

56

Question 51

Determine whether the load is energized or de-energized with the switch in the position shown. Also,identify whether the transistor is a depletion type or an enhancement type:

Load

+V

file 02338

Answer 51

The load will be energized as a result of this enhancement-type transistor being in the ”on” state.

Notes 51

Ask your students to explain how they figured out the state of the transistor in this circuit, and alsowhat function the double-pole, double-throw (DPDT) switch performs. Incidentally, this DPDT switchwiring configuration is quite common in electrical and electronic circuits!

57

Question 52

There is a problem somewhere in this electronic ignition circuit. The ”coil” does not output high voltageas it should when the reluctor spins. A mechanic already changed the coil and replaced it with a new one,but this did not fix the problem.

Chassisground

"Coil"

12 VDC

N S

Reluctor

TP1 TP2

TP3

TP4

Ignitionswitch

You are then asked to look at the circuit to see if you can figure out what is wrong. Using yourmultimeter, you measure voltage between TP1 and ground (12 volts DC) and also between TP2 and ground(0 volts DC). These voltage readings do not change at all as the reluctor spins.

From this information, identify two possible faults that could account for the problem and all measuredvalues in this circuit, and also identify two circuit elements that could not possibly be to blame (i.e. twothings that you know must be functioning properly, no matter what else may be faulted). The circuitelements you identify as either possibly faulted or properly functioning can be wires, traces, and connectionsas well as components. Be as specific as you can in your answers, identifying both the circuit element andthe type of fault.


1.

2.

• Circuit elements that must be functioning properly

1.

2.

file 02343

Answer 52


58

• Circuit elements that are possibly faulted1. Transistor failed shorted between collector and emitter2. Capacitor failed shorted

• Circuit elements that must be functioning properly1. Battery2. Ignition switch

Notes 52

Ask your students to identify means by which they could confirm suspected circuit elements, bymeasuring something other than what has already been measured.


59

Question 53

Sketch the shape of the output voltage waveform for this ”clipper” circuit, assuming an ideal diode withno forward voltage drop:

Vout

(ideal diode)

file 02352

Answer 53

Vout

(ideal diode)

Notes 53

This circuit is not difficult to analyze if you consider both half-cycles of the AC voltage source, one ata time. Ask your students to demonstrate this method of analysis, either individually or in groups, at thefront of the classroom so everyone can see and understand.

60

Question 54

When the pushbutton switch is actuated, the lamp energizes and remains energized even when theswitch is released:

Insert a second switch into the schematic diagram that allows someone to de-energize the lamp once ithas been ”latched” on by actuating the first switch.

file 02388

Answer 54

Notes 54


61

Question 55

A resistive DC load receives pulse-width modulated (PWM) power from a controller circuit, and anoscilloscope shows the load voltage waveform as such:

Vertical sensitivity = 1 volt/div

Coupling = DCProbe ratio = 10:1

Timebase = 20 µs/div

Calculate the duty cycle of this waveform, and also the average power dissipated by the load assuminga load resistance of 10.3 Ω.

file 02348

Answer 55

Duty cycle ≈ 58.3%Paverage ≈ 80 W

Follow-up question: which oscilloscope setup parameters (vertical sensitivity, probe ratio, coupling, andtimebase) are necessary for performing these calculations? Which parameters are unnecessary, and why?

Notes 55

Calculating the duty cycle should be easy. Calculating load power dissipation requires some thought.If your students do not know how to calculate average power, suggest this thought experiment: calculatingpower dissipation at 0% duty cycle, at 100% duty cycle, and at 50% duty cycle. The relationship betweenduty cycle and average power dissipation is rather intuitive if one considers these conditions.

If a more rigorous approach is required to satisfy student queries, you may wish to pose another thoughtexperiment: calculate the energy (in units of Joules) delivered to the load for a 50% duty cycle, recallingthat Watts equals Joules per second. Average power, then, is calculated by dividing Joules by seconds overa period of one or more whole waveform cycles. From this, the linear relationship between duty cycle andaverage power dissipation should be clear.

62

Question 56

A technician is troubleshooting a faulty optically-isolated TRIAC power switching circuit. The solenoidvalve is supposed to open up and pass liquid through it whenever the pushbutton switch is pressed, but itremains shut no matter what state the switch is in:

Solenoid valve

Pipe Pipe

To 120 VACpower source

+-

Battery TP1

TP2TP3

TP4

TP5TP6

Pinout of opto-TRIAC

Switch

Leaving the switch in its normal (”unpressed”) position, the technician measures 120 volts AC betweentest points TP5 and TP6, and 9 volts DC (normal for the battery) between test points TP1 and TP3.Based on these voltage measurements, identify two possible faults that could account for the problem andall measured values in this circuit, and also identify two circuit elements that could not possibly be to blame(i.e. two things that you know must be functioning properly, no matter what else may be faulted). Thecircuit elements you identify as either possibly faulted or properly functioning can be wires, traces, andconnections as well as components. Be as specific as you can in your answers, identifying both the circuitelement and the type of fault.


• Circuit elements that must be functioning properly1.2.

file 02402

63

Answer 56

Note: other failures and ”good” elements may exist!

• Circuit elements that are possibly faulted1. open TRIAC2. open switch

• Circuit elements that must be functioning properly1. 9 volt battery2. AC power source

Notes 56


64

Question 57

Calculate the DC voltage across the load connected to this regulated power supply circuit, and also theamount of current through the silicon transistor’s base terminal, given a load resistance of 120 Ω:

Plug 1 kΩ120 VAC

transformer

1000 µF

120 / 12

5.6 VLoad

β = 35

(120 Ω)

Vload = IB =file 02496

Answer 57

Vload = 4.9 V IB = 1.134 mA

Notes 57


65

Question 58

The following strobe light has a problem: the flash tube never flashes.

Q1

Q2

Q3

Q4

R1R2

C1

R3

R4

R5

R6

C2

T1

Flash tube

On/off

TP1TP2

TP3

TP4

TP5

TP6

6 V

Turning the flash rate control (rheostat R1) to the slowest position, you take two voltage measurementswith a voltmeter: at test point 3 (between TP3 and ground) you measure a voltage rhythmically pulsatingbetween about 1.5 and 4 volts DC. At test point 6 (between TP6 and ground) you measure 6 volts DC allthe time.

From this information, identify two possible faults that could account for the problem and all measuredvalues in this circuit, and also identify two circuit elements that could not possibly be to blame (i.e. twothings that you know must be functioning properly, no matter what else may be faulted) other than the 6volt battery and the on/off switch. The circuit elements you identify as either possibly faulted or properlyfunctioning can be wires, traces, and connections as well as components. Be as specific as you can in youranswers, identifying both the circuit element and the type of fault.


• Circuit elements that must be functioning properly (besides battery and on/off switch)1.2.

file 02630

Answer 58


• Circuit elements that are possibly faulted1. Resistor R4 failed open2. Resistor R5 failed open3. Resistor R6 failed open4. Connection between TP6 and collector of Q4 broken open

66

• Circuit elements that must be functioning properly (besides power supply and signal source)1. Rheostat R1

2. Resistor R2

3. Capacitor C1

4. Transistor Q1

Notes 58


67

Question 59

The following comparator circuit was built by an avid kite flyer, who wanted a circuit to alert him toabnormal wind conditions. Is this circuit configured to act as a high wind speed alarm, or a low wind speedalarm? Also, show which way the potentiometer wiper would have to be moved in order to obtain a higherwind speed alarm point.

−

+

+V

+VGen

Anemometer

Alarmthreshold

set

Alarm light

file 02664

Answer 59

This is a low speed alarm, and the wiper would have to be moved up to raise the wind speed thresholdvalue.

Notes 59


68

Question 60

Something is wrong with this regulated DC power supply circuit. The output is supposed to be +10.0volts, but instead it measures only about 1 volt:

Power plug

TP1 TP2

TP3

TP4

TP5TP6

120 / 12.6 VACtransformer

1 kΩ

470 µF

33 µF

−

+LM741C

VZ = 10.0 V

TP7

D1 D2

D3 D4

C1

C2

U1

Q1

D5

R1

Using your digital multimeter, you measure 15.3 volts between test points TP7 (red test lead) and TP4(black test lead). Note that VZ shown in the schematic is a specification for the zener diode, and not anactual voltmeter measurement. From this information, identify two possible faults that could account forthe problem and all measured values in this circuit, and also identify two circuit elements that could notpossibly be to blame (i.e. two things that you know must be functioning properly, no matter what else maybe faulted) other than the 120 volt AC power source, on/off switch, and fuse. The circuit elements youidentify as either possibly faulted or properly functioning can be wires, traces, and connections as well ascomponents. Be as specific as you can in your answers, identifying both the circuit element and the type offault.


1.

2.

• Circuit elements that must be functioning properly (besides 120 volt AC source, switch, and fuse)

1.

2.

file 02660

Answer 60


69

• Circuit elements that are possibly faulted1. Transistor Q1 failed open (base-to-emitter, collector-to-emitter, or both)2. Broken wire/trace between +V opamp terminal and Q1 collector terminal.3. Broken wire/trace between -V opamp terminal and ”-” output terminal.

• Circuit elements that must be functioning properly (besides 120 volt AC source, switch, and fuse)1. Transformer2. Rectifying diodes

Notes 60



70

Question 61

Is this opamp circuit inverting or noninverting?

−

+

Vout

VinR1

R2

file 02710

Answer 61

Inverting.

Notes 61


71

Question 62

Calculate the voltage gain of this opamp circuit (as a ratio, not decibels):

−

+

Vin

Vout

33 kΩ 12 kΩ

file 02715

Answer 62

AV = 2.75

It is okay if students express this answer as a negative quantity, since some texts teach the gain formulaAV = −

Rf

Ri.

Notes 62


72

Question 63

Calculate the voltage gain of this opamp circuit (as a ratio, not decibels):

−

+Vin

Vout

22 kΩ12 kΩ

file 02718

Answer 63

AV = 2.833

Notes 63


73

Question 64

Determine the minimum and maximum voltage gains possible with this circuit:

−

+Vin

Vout

1 kΩ 1 kΩ10 kΩ

AV (min) = AV (max) =file 02734

Answer 64

AV (min) = 1.09 AV (max) = 12

Notes 64


74

Question 65

Identify the names of the following logic gates:

file 02874

Answer 65

AND OR

INVERTER(or NOT)

NAND

Notes 65


75

Question 66

Determine the logic state (high or low) of each test point with the input switches in the positions shown,and also complete the truth table for the gate:

VDDVDD

TP1 TP2

TP3

TP4

A B Output

00

0 1

01

1 1

A

B(Output)

• TP1 = (high or low)• TP2 = (high or low)• TP3 = (high or low)• TP4 = (high or low)

file 03976

Answer 66

VDDVDD

TP1 TP2

TP3

TP4

A B Output

00

0 1

01

1 1

A

B(Output)

0

1

1

1

• TP1 = High• TP2 = High• TP3 = Low• TP4 = High

Notes 66


76

Question 67

Complete the truth table for the following relay logic circuit:

L1 L2

A CR1

CR2B

CR1

CR2

Indicator

A B Output

00

0 1

01

1 1

Truth table

file 02878

Answer 67

L1 L2

A CR1

CR2B

CR1

CR2

Indicator

A B Output

00

0 1

01

1 1

Truth table

1

0

1

1

Notes 67

77

Question 68

Something is wrong with this building alarm system circuit. The alarm siren energizes even when allthe windows and doors are shut. The only way to silence the alarm is to use the ”override” key switch:

TP1

TP2

TP3

TP4

Window switch

Window switch

Door switch

Door switch

VDD

R1 R2 R3 R4

VDD

Override

Normal

L1

L2

120 VAC

Siren

Fuse

Solid-staterelay

R5

TP5

TP6

TP7

TP8

(closed when shut)

(closed when shut)

(closed when shut)

(closed when shut)

TP9

TP10

TP11

TP12

TP13

TP14

SW1

SW2

SW3

SW4

U1

U2

U3U4

TP15

Using your logic probe, you measure a high signal at TP7 and a low signal at TP6 with all windowsand doors shut, and with the key switch in the ”override” position. From this information, identify twopossible faults that could account for the problem and all measured values in this circuit. Then, choose oneof those possible faults and explain why you think it could be to blame. The circuit elements you identifyas possibly faulted can be wires, traces, and connections as well as components. Be as specific as you can inyour answers, identifying both the circuit element and the type of fault.


1.

2.

• Explanation of why you think one of the above faults could be to blame

file 02882

Answer 68

Note: the following answers are not exhaustive. There may be more circuit elements possibly at fault!

78

• Circuit elements that are possibly faulted1. SW1 contacts not closing (dirty or worn)2. SW2 contacts not closing (dirty or worn)3. Broken wire between SW1 and TP14. Broken wire between SW2 and TP25. Broken wire between SW1 and ground6. Broken wire between SW2 and ground7. U3 output stuck high8. U1 output stuck high

Notes 68



79

Question 69

A water-cooled generator at a power plant has two sources of cooling water flow, each source equippedwith a flow switch that returns to its normally-open status if the water flow through the pipe stops. If eitherwater source ceases supplying cooling water to the generator (for whatever reason), that flow switch will de-actuate and turn the warning light on. This is all that is required, as the generator will still receive adequatecooling from only one source. If both water sources cease supplying water, however, a ”trip” solenoid willenergize to shut down the generator before it overheats:

L1 L2

Cooling waterflow (source A)

Cooling water

CR1

CR2flow (source B)

CR1

CR1

Warning light

CR2

CR2Shut-down

solenoid

TP1 TP2

TP3 TP4

TP5 TP6

TP7

TP8 TP9 TP10

One day the warning light comes on, but there is still cooling water to the generator so it does not shutdown. You are asked to determine what the problem is. Using your voltmeter, you measure 115 volts ACbetween TP2 and L2, and 0 volts AC between TP4 and L2. From this information, identify two possiblecauses that could account for the warning light status and all measured values in this circuit. Then, chooseone of those possible causes and explain why you think it could be to blame. The possible causes you identifycan be non-electrical as well as electrical in nature. Be as specific as you can in your answers.

• Possible causes

1.

2.

• Explanation of why you think one of the above possibilities could be to blame

file 02924

Answer 69

Note: the following answers are not exhaustive.

80

• Possible causes1. Cooling water source ”B” actually ceased flowing2. Switch contacts for flow switch ”B” failed open3. Broken wire between TP3 and TP4

Notes 69


81

Question 70

Draw the necessary wire connections to build the circuit shown in this ladder diagram:

Ladder diagram:

M1

M1 Motor

L1 L2

Pushbutton switch

Illustration showing components:

RelayMotor

L1 L2

(Dashed lines represent connectionsbetween relay terminals and socket

screw lugs, hidden from sight)

Pushbutton switch

file 03209

82

Answer 70

Note: this is not the only valid solution!

RelayMotor

L1 L2

Pushbutton switch

Notes 70


83

Question 71

Complete this ladder diagram for a forward/reverse three-phase motor control circuit, complete withinterlock contacts to prevent both M1 (forward) and M2 (reverse) contactors from simultaneously energizing:

L1 L2

motor

M1 OL

M2

Forward

Reverse

3-φpower

Be sure to include all these details:

• Proper placement of M1 (forward) and M2 (reverse) coils• Proper placement of overload contact (OL)• Proper placement of interlock contacts• Proper wiring of M1 and M2 contacts to motor

file 03211

84

Answer 71

L1 L2

motor

M1 OL

M2

Forward

Reverse

3-φpower

OLM1

M2

M2

M1

Notes 71


85

Question 72

Convert the octal number 344 to decimal form.file 02965

Answer 72

3448 = 22810

Notes 72

This question is intended for exams only and not worksheets!.Nothing to comment on here!

86

Question 73

Convert the decimal number 35 to hexadecimal form.file 02968

Answer 73

3510 = 2316

Notes 73

This question is intended for exams only and not worksheets!.Nothing to comment on here!

87

Question 74

Identify the logic states of all four test points (TP1 through TP4) when the correct pushbutton switchis pressed to reset the latch circuit:

VDD

Set

Reset

Q

Q

TP1

TP2

TP3

TP4

TP1 = TP3 =

TP2 = TP4 =file 03019

Answer 74

TP1 = 0 (low) TP3 = 1 (high)

TP2 = 1 (high) TP4 = 0 (low)

Notes 74


88

Question 75

An electronic counter is installed at the entrance doorway of a large convention room to count thenumber of people entering. As each person passes through the doorway, their body interrupts a light beamfrom an infra-red LED to a matching phototransistor, sending a pulse to the counter IC (LED light strikingthe phototransistor Q1 turns it on, and the absence of LED light turns it off). Every tenth person thatenters is supposed to win a prize, so the job of this circuit is to ring a bell for every tenth person who enters(at the ”terminal count” state). The problem is, the bell was working just fine a few hours ago, but hasn’trung ever since. Some of the convention attendees are starting to get angry, demanding door prizes.

Q0 Q1 Q2 Q3

D0 D1 D2 D3

Clk

PL

CETCDecade counter

RST

Gnd

TP1

TP2

TP3 TP4 TP5 TP6

TP7

Solenoid coil

BellHammer

TP9

TP10

Light beamLED1Q1

U1

Q2

D1

TP11TP12

+5 V

TP8

12 V battery

R1 R2

R3

TP13 TP14

TP15

You begin troubleshooting by taking voltage measurements while people randomly enter the room(breaking the light beam as they do). Using your voltmeter, you measure a constant +5 volts at TP3 withrespect to ground. This voltage measurement does not change, no matter how many people pass throughthe light beam. You then connect your voltmeter between TP12 and ground, measuring approximately 0volts all the time whether or not someone is passing through the doorway.

From this information, identify two possible causes that could account for the problem and all measuredvalues in this circuit. Also, identify the next logical test point(s) from those shown on the schematic thatyou would check with a voltmeter, and why you would check there. Note that there may be more than onecorrect answer to this part of the question!

• Possible causes of the problem1.2.

• Next logical test point(s) to check, with reason why you would check thereNext point(s):

Reason:

file 03097

Answer 75

Note: the following answers are not exhaustive. There may be more circuit elements possibly at faultthan what is listed here!

89

• Possible causes of the problem1. Phototransistor Q1 failed open2. LED failed open3. Resistor R1 failed open4. Wire or connection broken between TP1 and LED, or TP1 and Q1

5. Resistor R2 failed shorted

• Next logical test point(s) to check, with reason why you would check there- TP14 to ground, to see if Q1 is being supplied power- TP13 to ground, to see if the LED is being supplied power

Notes 75


90